Environmental Protection Agency (EPA) regulations require that industries continuously monitor the emission of atmospheric and water pollutants generated as by-products of industrial activity. Of central concern is the emission of fly ash from the stacks of coal-fired boilers. Determination of the weight of fly ash produced as by-product requires two separate measurements: (1) a measurement of the particulate density of the stack gases; and (2) a measurement of the volumetric flow rate of the gases passing through the stack. As defined herein, "opacity" is the fraction of incident light which is lost in transmission through an optical medium. Currently, the standard method of measuring particulates contained in stack gases is optical transmissometry where the opacity of stack gases is determined. The product of particulate density and flow rate yields the weight per unit time of fly ash emitted into the environment from a unit cross-sectional area of the stack.
Optical measurements of particle velocity are widely used in studies of gas flows and also in particle dynamics. One well-known velocity measurement technique is laser Doppler velocimetry, also known as laser Doppler anemometry. According to this approach, particles or other elements simultaneously scatter light from two coherent beams, each of the beams having different angles of incidence. A photodetector receives the light and generates a frequency representing the heterodyne difference in Doppler shift frequencies produced by the motion of the particles relative to the beams. Laser Doppler velocimetry is useful but requires careful alignment of the beams as well as maintenance of beam coherency.
Another well-known velocity measuring technique is time-off-light velocimetry, also referred to as transit time, two-spot, or two-focus velocimetry. According to this method, two beams of electromagnetic radiation with radially symmetrical intensity distributions are directed through a particle sampling volume. A particle, when passing through both beams, generates two pulse signals corresponding to each beam. A timing signal is initiated, coincident with the first pulse, and terminated, coincident with the second pulse, for an open "time-of-flight" measurement over a known distance, thus yielding a particle velocity. This approach provides better signal-to-noise ratios than laser Doppler velocimetry for certain applications and does not require laser light beam coherence. Further, since time-of-flight velocimetry involves measuring time rather than frequency, the typical time-off-light application can employ lower cost, signal processing circuitry.
However, time-of-flight velocimetry is subject to error. For example, a particle can produce a signal sufficient for detection when passing through one of the beams, but not the other, either due to the particle's trajectory or the particle's size being at the borderline of detection by the other beam. This occurrence can lead to an erroneous time-of-flight measurement when a signal from the arrival of a second particle is mistakenly identified as the departure signal of the first particle.
Several emissions monitoring systems, currently available, can separately determine the opacity and flow rate of stack emissions. A major shortcoming of such systems is that opacity is determined using a stand-alone, optical-based monitoring system, while flow rate is determined using an independent, acoustically-based monitoring system. These two systems are typically supplied by different companies and use technologies that are dissimilar. One measurement necessarily requires the other measurement in order to determine emission of fly ash, and the costs of not just one but both monitoring systems must also be considered.
Additionally, emissions monitoring systems currently on the market require four stack penetrations: two stack penetrations for the opacity monitor, and two more stack penetrations for the flow monitor. Because current flow monitors determine flow rate of stack gases by measuring a differential time-of-flight of two ultrasound waves (e.g., one wave traveling with the flow, the other wave travelling against the flow), the two stack penetrations for the flow monitor must be displaced along the length of the stack.
Current emissions monitoring systems also pose another problem in that the current systems transfer opacity and flow rate data collected by stack mounted units to a remote data processing station using electrically conducting cable. Utilization of electrically conducting cable for data transfer results in systems with significant susceptibility to damage by lightning. Not only can lightning destroy stack mounted units, electrical surges conducted to remote monitoring stations by the conducting cable can also destroy the remote equipment used to analyze and store collected data. As a result, lightning can damage and disable the typical current emissions monitoring system in its entirety.
Damage from lightning to current emissions monitoring systems can occur even if the stack mounted unit does not experience a direct lightning hit. Stacks are often separated from remote monitoring stations by distances of a thousand meters or more. Lightning strikes in the vicinity of a stack can produce voltage surges on the conducting cable connecting the stack-mounted units to the remote monitoring site that are large enough to damage electrical equipment.
What are needed are an optical based opacity and flow monitoring system for pollutant emissions and a method of measuring opacity and flow rate of pollutant emissions. Also, an opacity and flow monitoring system for pollutant emissions is needed that is substantially immune to electrical surge resulting from lightning strikes on or near an industrial emissions stack.